1. Field of the Invention
The present invention relates to surface scanning systems. More particularly, the invention relates to systems and methods of semiconductor surface scanning using scatterometers.
2. Description of Related Art
Current ellipsometers and reflectometers predominantly use point-to-point stages and so called move-acquire-measure (MAM) process to perform metrology on semiconductor wafers or other scanned surfaces. The “move” portion involves point-to-point motion of a wafer carrying stage, and includes acceleration of the stage, followed by deceleration. Because of practical mechanical limitations imposed on acceleration rate, the typical time to move from point to point cannot be less then a few tens of milliseconds. Once the wafer (sample) is positioned for the optical system to start the next acquisition, the “acquire” portion of the process commences in which the optical system acquires sample data. In the case of polarized measurements, that typically takes at least multiple milliseconds, more if higher sensitivity is desired, and even more if the system has moving parts such as a rotating polarizer and/or an analyzer. The “measure” portion involves data analysis and may commence concurrently with the start of a subsequent MAM cycle. Overall, practical limitations and sensitivity requirements may limit MAM times to hundreds of milliseconds. For example, assuming 200 ms MAM and 300 measurements per wafer sample plan, a single wafer will require 60 seconds to process, which equates to 60 wafers per hour (wph) throughput. Therefore, the practical limit on the number of samples per wafer may be estimated to not exceed ˜1000 measurements to allow better than 20 wph throughput.
An advantage of the point-to-point method is the ability to measure within a specified box, which may have dimensions on the order of 10 μm or less. Ellipsometers and reflectometers also have the unique ability to perform non-destructive measurements of film thicknesses, dispersion coefficients (n and k), and critical dimensions of gratings (OCD). While such point-to-point systems provide better sensitivity and the ability to measure specific locations, such systems are also inherently limited by the Move-Acquire-Measure (MAM) time per single measurement. Typical measurement sample plans may not exceed a few tens of locations per wafer to stay within reasonable wafers-per-hour (WPH) throughput range. The limitation in measurement locations (scanning area on the sample) arises because of limited acceleration and speed of a sample moving system (the overhead between sample measurements) in combination with the time to make the actual measurement on the sample (e.g., time for point-to-point data acquisition). Thus, it may be beneficial for a semiconductor manufacturer to have a complete map of a wafer without sacrificing throughput.
Alternatives to the point-to-point systems are systems that record all necessary signals simultaneously with the quality of the signal limited by exposure time. U.S. Pat. No. 7,121,357 to Meeks, which is incorporated by reference as if fully set forth herein, discloses an example of such a system. Such systems would predominantly make measurements in a given location on a wafer with the wafer shift (during measurement) similar or less than the optical spot size. The wafer shift being similar or less than the optical spot size, however, limits the mapping size on the wafer for data acquisition. In addition, simultaneously recording all the signals may put severe restrictions on available engineering solutions. For example, rotating polarizer/analyzer/compensator ellipsometers (RPE/RAE/RCE) and their combinations may be excluded from being used in a system that records all necessary signals simultaneously because they are non-simultaneous measurement systems. RPE/RAE/RCE ellipsometers, however, provide best-in-class sensitivity. Future demands for higher sensitivity may require the use of additional Mueller matrix elements, which would further increase the desire for using non-simultaneous measurement systems while providing higher scanning system throughput.
It may also be desired for a scanning system to provide capability to collect specular reflection order information from a sample via multiple channels. Collecting specular reflection order information via multiple channels may provide improved sensitivity, improved throughput, and possibly provide the ability to resolve small features on the wafer. In addition, small surface defects, such as scratches, may be detectable.
U.S. Pat. Appl. Pub. No. 2008/0014748 to Perry, which is incorporated by reference as if fully set forth herein, describes a multichannel reflectance analyzing system that relies on a bundle of fibers. A problem in the disclosed multichannel reflectance analyzing system is that it relies on a bundle of intermixed illumination and collection fibers. It may be implemented in two ways. A first way of implementation is imaging the sample surface on the fiber entrance. Since collection fibers constitute only a portion of the bundle, however, the system will only provide a subset of surface information and would not be suitable for detection of surface defects. That behavior may be suitable for the applications described in the application but would not be acceptable for a wafer inspection system. A second way of implementation is defocusing the system. When the system is defocused, however, the optical throughput suffers and the system may not provide desired surface resolution (e.g., the system may not be sensitive to surface defects).